The invention relates to an electroacoustic transducer comprising a capacitive acoustic element and at least two switches for controlling the voltage acting on the element, in which case the switches are arranged to control the voltage acting on the element by controlling the on and off times of the switches.
The coefficient of efficiency of sound reproducers based on magnetic loudspeakers is typically very low, about 0.5%, for example. It is known to control magnetic loudspeakers by so-called chopper amplifiers in which case the efficiency of the amplifier is reasonably good, but as the resistance of the coil of the loudspeaker is fairly great, it causes a great power loss and the total efficiency of the sound reproducer will thus be very low.
DE-2324211 discloses a capacitive acoustic element but the reference cited does not disclose the control arrangements of the element. U.S. Pat. Nos. 4,207,442, 4,286,122 and 5,161,128 also disclose a capacitive acoustic element and various control switchings and arrangements of the element. All the solutions mentioned above have it in common that the coefficient of efficiency will not be very good by means of them.
The object of the present invention is to provide an electroacoustic transducer whose coefficient of efficiency will be very good.
The transducer of the invention is characterized in that an inductance is connected to at least one electrode of the acoustic element, through which inductance voltage is arranged to act on the acoustic element, and that the transducer comprises a capacitance that together with the inductance forms an electrical circuit in such a manner that the capacitance and the inductance together operate as an energy storage for storing energy unconverted into acoustic power.
The essential idea of the invention is that the capacitive acoustic element is controlled by means of at least two fast switches, in which case by controlling the off and on times of the switch, the voltage acting on the transducer is controlled. A further essential idea is that an inductance is connected to at least one electrode of the acoustic element, through which inductance voltage is arranged to act on the acoustic element. The inductance together with the capacitance of the transducer forms an oscillating circuit in such a manner that the inductance and capacitance in question are able to store energy unconverted into acoustic energy and supply it back to the transducer. The energy stored into the acoustic element is transferred almost without loss e.g. to another block of the element or to an independent storage capacitor and back to the element. The idea of one preferred embodiment is that the switches are controlled by pulses whose width is determined by means of the difference of an audio signal and the voltage of the transducer, that is, pulse width modulation is used. Furthermore, the idea of a second preferred embodiment is that the acoustic element is formed of a serial connection of two capacitors, at least one of which is acoustically active.
The advantage of the invention is that the coefficient of efficiency of the equipment is very good as only that amount of energy will be consumed that the transducer emits out as acoustic power and the portion used for the switch losses of control electronics.
A separate auxiliary capacitor will not be needed for the electrical circuit when the acoustic element comprises two capacitors.